1. Field of the Invention
The present invention relates generally to organic positive temperature coefficient (PTC) thermistors used as face-like heating devices, and more particularly, to an organic positive temperature coefficient thermistor having an improvement in the structure of the electrodes that are formed on a sheet exhibiting a positive temperature characteristic of resistance.
2. Description of the Prior Art
For example, a material obtained by throughly mixing polyolefin such as polyethylene with conductive particles such as metal powder, carbon black or graphite exhibits a positive temperature characteristic of resistance. An organic positive temperature coefficient thermistor using a sheet made of this material can be used as a flexible face-like heating device.
In the above described organic positive temperature coefficient thermistor, the following electrode structure is formed on one surface of the sheet exhibiting a positive temperature characteristic of resistance. More specifically, this electrode structure has a pair of power supplying portions arranged opposed to each other and separated by a predetermined distance, and a plurality of branch-shaped electrodes electrically connected to the power supplying portions, respectively, and arranged so as to be inserted between each other (interdigitated) between the power supplying portions.
A face-like heating device utilizing this organic positive temperature coefficient thermistor can be kept at a constant temperature and can automatically control the heat generating temperature under abnormal conditions because it has a self-temperature control function. Consequently, it is superior in safety to a face-like heating device using a nichrome wire and a metal foil.
However, the organic positive temperature coefficient thermistor has the disadvantage in that it is very difficult to switch the temperature, that is, to change its output temperature because it has the above described self-temperature control function.
In the conventional organic positive temperature coefficient thermistor, the output is switched in the following manner. More specifically, a plurality of electrode structures as described above are formed on the sheet exhibiting a positive temperature characteristic of resistance, to construct a plurality of separate heat generating circuits. The electrical connections to the plurality of heat generating circuits can then be switched. That is, the heat generating circuits which are placed in the on state can be selected, so that the area in which heat is generated can be reduced to, for example, one-half or one-third of the area in a case where all the heat generating circuits are caused to generate heat.
In the above described structure, however, heat is generated on only part of the surface of the sheet exhibiting a positive temperature characteristic of resistance, that is, the portion where heat is generated and the portion where heat is not generated are completely divided. Accordingly, heat cannot be uniformly generated in the entire region where heat is desired to be generated.